Techniques for quantitatively evaluating the internal structure of optical fibers and fiber preforms are described in U.S. Pat. Nos. 4,161,656 and 4,181,433. Because these techniques, and the one to be described hereinbelow are equally applicable to both optical fibers and fiber preforms, either term or the term "fiber/preform" will be used hereinafter to designate both.
In the '656 patent, the fluorescence induced in the index-modifying dopants used to grade the index profile is measured. Alternatively the absorption by the dopants of the fluorescence-inducing ultraviolet (uv) is measured. In the '433 patent, the density distribution of the incident light as, focused by the fiber/preform core region, is measured. For accurate results, both of these techniques require that the radiation incident upon the core of the fiber/preform be a well collimated, uniform wave. However, because of the strong focusing action of the surrounding cladding it was deemed necessary to immerse the fiber/preform in an indexing-matching fluid in order to maintain the planar nature of the illuminating beam, and to observe the entire core region. Without such immersion, the strong focusing action of the cladding makes it virtually impossible for an observing lens of reasonable size to collect the sharply diverging rays that traverse the core.
Another problem associated with the use of matching fluids is the nonavailability of conveniently usable, low-loss fluids having high indices (i.e.,&gt;1.6). The problem is that the heavier fluids, having the requisite high indices, tend to be toxic and therefore more difficult to handle. Finally, even when the required matching fluids are available, their use is not only an inconvenience, but there is always the possibility of scratching the preform and damaging it when it is being wiped clean of the fluid. Advantageously, measuring techniques which avoid the use of matching fluids are preferable.